This invention relates generally to devices which determine the coefficient of friction between two surfaces, and, more particularly, to devices which may be used to determine the coefficient of friction between surfaces involved in automobile accidents.
In order to determine the speed at which an automobile was traveling at the time of its brakes locking, a police officer or investigator must determine the coefficient of friction between the vehicle and the surface or surfaces over which the vehicle skidded. A simple formula is used to calculate this data. For a single surface, the formula is f=s.sup.2 /30(d) where f=coefficient of friction, s=speed of vehicle in miles per hour, and d=distance of skid in feet. A value for the coefficient of friction f between two given surfaces can be calculated by the amount of force required to move one surface at a steady rate across the other. If W represents the weight of the moving body and P represents the force required to move the body, f=P/W. Where the type of surface changes over the length of the skid (either the vehicle or the roadway surface), a composite formula must be utilized: s.sup.2 =30(f.sub.1 d.sub.1 +f.sub.2 d.sub.2 +f.sub.3 d.sub.3 . . . ), where the subscripts indicate the varying values of f and d for each different combination of surfaces. The type of roadway surface can vary over the length of the skid (e.g. asphalt, concrete, gravel, soil, grass, etc.). Just as the roadway surface may vary, so may the vehicle surface (e.g. tires or some metal surface of the vehicle).
An accident investigator, in order to accurately determine the variables in the above formulae, should take measurements at the scene of the accident under the same conditions as those of the accident. One method which has been employed is accomplished by the investigator skidding his own vehicle over the surface, taking measurements to determine the coefficient of friction for the surface and then applying the coefficient to the data from the accident. This method is often inaccurate and in many cases impossible because of safety considerations.
As result of these problems, devices have been designed for determining the coefficient of friction at the scene of an accident. Examples of such devices may be found in the following patents:
______________________________________ U.S. Pat. No. Inventor Title Issue Date ______________________________________ 3,301,039 Kummer Skid Resistance 1/31/67 Drag Tractor 3,538,742 Benning Friction 11/10/70 Measurement 4,130,008 Broshears Device for 12/19/78 Measuring Friction & Distance 4,144,748 Vinogradov, Device for 3/20/79 et al Determining Coefficient of Adhesion of Pneumatic Wheel Tires of Transport Vehicles to Road Pavement 4,187,714 Cox, et al Surface Friction 2/12/80 4,315,426 Brandon Friction Co- 2/16/82 efficient Measurement from a Moving Vehicle ______________________________________
Some of these devices are trailer-mounted (e.g. U.S. Pat. Nos. 3,538,742 and 4,144,78) and as such are difficult to use off standard roadway surfaces. These devices are also usually too great in size to be carried in a police car. Other devices, such as U.S. Pat. No. 4,315,426, must be added to a vehicle, resulting in unnecessary expense as well as being difficult or impossible to operate off the roadway. Also, because the trailered and add-on devices have linkage arrangements for force measurement, accuracy may be lost.
More simplistic devices have been designed which may be carried in the vehicle of an officer investigating at the accident scene. Examples of these devices are the Broshears patent (U.S. Pat. No. 4,130,008) and the Cox patent (U.S. Pat. No. 4,187,714). These devices have proven inaccurate due to the "gearing effect", or rotational force, which results when the device is pulled at a point above the center of mass of the device. In order to permit consistent and accurate readings, the pulling force should be applied at the center of mass of the device in a direction parallel to the road surface or other surface to be tested. The accuracy of the Cox device is further impaired by the squared edge of the friction surface (usually a tire tread) which will tend to dig into the roadway when the rotational force is applied. In order to counteract the rotational force, the devices can be pulled at an angle other than parallel to the roadway. However, inaccurate and inconsistent readings will also result from such an application of force.
Compounding the above problems is the fact that, in order to apply the prior art devices in situations where something other than a tire has skidded (such as a vehicle on its side), the friction surface on the device must be changed.
The need therefore exits for a friction measuring device which is compact, easy to use, distributes weight evenly, and yields consistently accurate results.